Sam Harris has some pretty good takes on why Trump is so bad but a lot of it comes down to just personally hating the guy. Harris :

“ There is never a moment where I find Trump persuasive. When I look at him I see a man without any inner life. I see the most superficial person on Earth. This is a guy who has been totally hollowed out by greed and self regard and delusion. If I caught some sort of brain virus and I started talking about myself the way Trump talks about himself, I would throw myself out a fucking window. ”